Thermal-spectrum molten salt reactors (MSRs) and fluoride salt–cooled high-temperature reactors (FHRs) include graphite materials in contact with molten salt. Graphite materials, including fuel elements (in FHRs), neutron reflectors, coolant channels, and other core internals, structural materials and lining materials (in MSRs and FHRs) will be exposed to molten salts and the cover gas above molten salt. It is important to understand if the exposure of graphite to fluoride salts impacts the chemistry of the graphite, its ability to scavenge tritium from the salt, and its sensitivity to degradation under air ingress and under irradiation. Two nuclear graphite IG-110 samples have been exposed to molten FLiBe salt and cover gas in the vicinity of molten FLiBe, at 700°C for 240 hours. Based on XPS analysis, formation of fluorinated sites in graphite (i.e. formation of C-F bonds) is observed in both liquid and gas-phase exposure conditions. FLiBe reduction by perfect graphite is not thermodynamically favorable but the presence of reactive carbon sites might drive the reduction. The sp3/sp2 ratio of the carbon in graphite was also observed to increase by a factor of two after exposure to the molten salt. The increase in the sp3/sp2 ratio may indicate that fluorination introduces new defects in the lattice, i.e. new reactive sites become available for tritium chemisorption. This presentation provides a comparison of the fluorination and fluorination effects between the salt-exposed and salt-vapor-exposed graphite samples.

Chemical modifications of graphite exposed to molten 2LiF-BeF2 (FLiBe) and to the vapor space above the salt

ACS Fall 2020 Virtual Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.